What is Optical Attenuation?

For any fiber optic cable to be highly effective at transmitting signals and/or data, the light passing through the fiber optics needs to be coherent, i.e. the light beam needs to be tight so that it doesn’t lose power through some of the light spreading out radially. Whilst fiber optics have been designed to minimize the radial distribution of light once it has been emitted from the source, there are still ways in which the power of the fiber optic can be reduced. This is known as optical attenuation, and it arises due to a number of variables. In this article, we look at what optical attenuation is and the different factors that contribute towards it.

Attenuation, in any form, is the loss of power (or flux intensity) over a long distance when the signal in question is transmitted through a medium. In fiber optics, this is the loss of light intensity through the fiber optic between the source and the receiver. Because fiber optics are highly advanced transport networks, the levels of attenuation aren’t as high as other signals/transport media which experience attenuations, but it can still occur. There are few main ways why fiber optic signals are privy to attenuation, and these are absorption, scattering, macrobending loss, microbending loss, and interface inhomogeneities.

Absorption

Losses in light intensity from absorption mechanisms are primarily due to the presence of metal ions in the glass components of a fiber optic cable. Metal ions usually arise in small concentrations during the production process, but metal ions at the parts per million level can even affect the absorption properties of the glass. These impurities can either be intrinsic, i.e. within the material itself, or extrinsic, which arise when there is water vapor in the glass components. Even the presence of hydrogen gas can influence the absorption characteristics of the fiber, as it can either infiltrate the fiber and be in the path of the light wave, or it can form hydroxyl ions, both of which increase the light absorption of the fiber.

Ultrapure glass will absorb the same amount of light relative to its thickness. However, this changes when the glass is not pure and contains metal ions because this extra degree of absorption can cause the intensity of the light signal to drop and will contribute to optical attenuation of the light beam.

Scattering

There are many different scattering mechanisms that can affect the light signal. Scattering occurs when the light waves interact with particles in the fiber optic which causes the light to lose energy in directions other than the propagating direction. In a fiber optic, the light can scatter through both elastic and inelastic means. Examples of elastic scattering in a fiber optic include Rayleigh Scattering and Mie scattering, whereas light can scatter in a non-elastic manner through Brillouin Scattering and simulated Raman scattering.

In terms of what causes these scattering mechanisms to occur within a fiber optic, the answer is different for each type of scattering. Rayleigh scattering occurs due to small-scale inhomogeneities present in the glass components in the fiber and can cause attenuation if the scattered light travels in a direction away from the propagation—if the light stays in the ‘forward’ direction, it doesn’t cause attenuation. In the case of Mie scattering, it is also caused by inhomogeneities, but this time it is mostly specific to the cladding and the cladding interface.

For the inelastic methods, Brillouin scattering is caused by nonlinearities in the medium. This can create scattered low-frequency photons in the opposite direction to the propagating wave, and this contributes to the attenuation of the light wave. Simulated Raman scattering, on the other hand, arises from molecular vibrations within the glass. Simulated Raman scattering produces high-frequency scattered photons, but it is rare that simulated Raman scattering contributes to attenuation of the light wave because most scattered photons travel in the same direction as the propagating wave (but some can travel in the opposite direction).

Macrobending and Microbending

Macrobending and microbending losses occur in areas where the fiber optic cable is bent. Macrobending refers to fiber sections that have a large radius of curvature relative to the fiber diameter, whereas microbending refers to areas of the cladding that exhibit localized bending. Both curvatures can cause lowering of the light wave intensity.

Bending the optical fiber can lead to huge intensity losses, especially if the radius of curvature of the bend is smaller than several centimeters. Microbends are very small bends at the interface of the cladding and can come about due to mechanical stress on the fiber or during the production of the fiber. These irregularities at the cladding interface can cause small amounts of optical attenuation, but not at the scale that macrobending can if the bend section is too tight.

Interface Inhomogeneities

The final way that a fiber optic can experience attenuation is through impurities, either at the cladding interface or in the fiber buffering. In both cases, these impurities can cause geometric inhomogeneities within the fiber which leads to optical attenuation occurring. However, as fibers have advanced over the years, and have become more defect-free, impurities play a less significant role in the occurrence of optical attenuation compared to other intensity-loss mechanisms.

Sources and Further Reading

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